Methacrylic resins are excellent in transparency, weatherability and scratch resistance and are widely used particularly for their transparency as molding materials for optical parts which demand precise molding, such as optical disks and lenses, or for lighting.
The methacrylic resin has recently found use as light conducting plate for back lighting in liquid crystal displays, etc., and the severity of requirements for transparency, color tone and freedom from foreign matter has been increasing. A light conducting plate as herein referred to is a part constituting a side-lighted type surface lighting unit mainly used for back lighting of liquid crystal displays. The side-lighted type surface lighting unit is disclosed in JP-A-57-128383 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"). As shown in FIG. 1, the lighting unit of this type comprises a light source, such as a cold cathode gas discharge tube, a hot cathode gas discharge tube, a light bulb or an LED, placed at the side of a light-emitting surface. The light emitted from the light source enters the light conducting plate from the side, changes its direction by a light scattering means provided on the light-reflecting surface, and comes out through a diffusing film via a polarizer to a viewer side. The light scattering strength of the light conducting plate can be designed appropriately in conformity with the distance from the light source or the position of the reflecting surface so as to provide a uniform surface lighting unit. Because such a side-lighted type surface lighting unit has its light source provided at the side of the light conducting plate thereby to contribute to reduction in thickness and weight of the whole display, it has been recently used as a backlight of liquid crystal displays used in laptop or notebook word processors or personal computers. Since such portable equipment is demanded to work for a long period of time with batteries contained therein, the side-lighted type surface lighting unit used for back lighting is required to have a low power consumption.
In FIG. 1 is shown a schematic illustration of a side-lighted type surface lighting unit used as a backlight of a liquid crystal display. Liquid crystal panel 1 possesses a function of controlling a light transmission at a desired position on the screen thereby forming letter or image information. Since liquid crystal panel 1 per se does not emit light, a backlight 2 is needed for obtaining a clear image. As light source 25, a cold cathode fluorescent tube which consumes relatively low power of electricity and yet provides sufficient brightness is of frequent use.
Light conducting plate 21 is a transparent resin plate having provided light-scattering means 22 on the light-reflecting surface thereof. Light-scattering means 22 is formed by injection molding using a mold having a prescribed pattern or by applying a white ink to the light-reflecting side with gradation by, for example, printing. Light conducting plate 21 can have a flat shape as with the case of FIG. 1, a wedge shape having a thickness decreasing with the distance from the light source, and other special irregular shapes. Numeral 23 is a reflecting film which functions to reflect light having passed through light conducting plate 21 to a viewer side thereby to improve light utilization. Numeral 24 is a frosted glass-like cloudy film called a diffusing film. Since light-scattering means 22 has a dot or stripe pattern, diffusing film 24 is effective to prevent such a pattern from being seen through liquid crystal panel 1. At the same time, a surface lighting unit which scatters light uniformly can be obtained by blurring the pattern by diffusing film 24. Numeral 26 is a lamp cover, which is used for efficiently leading light emitted from light source 25 into light conducting plate 21. The arrows in FIG. 1 indicate the light proceeding direction.
Thus, a light conducting plate is set on the back side of a liquid crystal panel and used in a shielded state, little light from the outside is let in.
With the recent spread of color and/or wide liquid crystal displays, the demand for a surface lighting unit achieving uniform light emission at a high luminance and with freedom from unevenness in luminance or color has been increasing. To meet this demand a large number of techniques have ever been disclosed.
For example, light conducting plates having provided unevenness so as to increase the amount of scattered light with the distance from the light source are disclosed in (1) JP-A-U-61-157986 (the term "JP-A-U" as used herein means an "unexamined published Japanese utility model application"), (2) JP-A-U-62-87315, (3) JP-A-U-63-43186, (4) U.S. Pat. No. 4,059,916, (5) U.S. Pat. No. 4,937,709, (6) U.S. Pat. No. 5,134,549 and (7) JP-A-U-5-79537. A technique for molding such a light conducting plate is disclosed in (8) JP-A-4-52286, in which a mold having a prescribed pattern formed by photoetching is used. (9) JP-A-2-165504 and (10) U.S. Pat. No. 5,074,675 disclose a light conducting plate which increases the amount of scattered light with the distance from the light source, in which the surface at the bottom facing the light-emitting surface is provided with grooves having a specific section designed to efficiently reflect the light from the light source to the viewer side, and the interval and the depth of the grooves increase with the distance from the light source so as to increase the amount of scattered light accordingly. Further (11) JP-A-U-60-94605, (12) U.S. Pat. No. 4,998,809 and (13) U.S. Pat. No. 5,040,098 disclose a technique for achieving both reductions in thickness and weight and increase in luminance, which comprises forming light-scattering grooves on the bottom surface facing the light-emitting surface and shaping the light conducting plate itself into a wedge having its thickness decreased with the distance from the light source.
In addition, it has been proposed to disperse light-scattering particles in the inside of a transparent light conducting plate as described in (14) JP-B-39-1194 (the term "JP-B" as used herein means an "examined published Japanese patent application"), (15) JP-A-53-36199, (16) JP-A-54-105562 and (17) JP-A-U-56-58678. In this connection, (18) JP-A-2-221924 and (19) JP-A-2-221925 disclose a process for producing a light conducting plate in which light-scattering particles are non-uniformly distributed by taking advantage of a difference in rate of precipitation.
The above-cited patent and utility model publications have a mention that the materials suitable as light conducting plate include glass, methacrylic resins, styrene resins, polycarbonate resins, and the like, but give no particulars on the characteristics required of such transparent materials.
With respect to resins for optical disks, various techniques for obtaining a transparent methacrylic resin suffering from little discoloration have been proposed. For instance, (20) JP-B-7-132598 discloses a technique for obtaining a colorless and transparent methacrylic resin having a high optical purity by specifying the molecular weight, pyrolysis index and yellowness index of a methacrylic resin and the volatile content and fine foreign matter content in the resin.
In high-speed molding of thin-walled articles, silver streaking often occurs due to volatile components of a resin, such as the monomer or the water content. In order to minimize the volatile content of a resin as a countermeasure for silver streaks, it has been a practice to remove the volatile matter in high vacuum at the time of extrusion and pelletization. (21) JP-A-58-154751 describes that active reduction of the monomer content in a resin is effective to suppress occurrence of silver streaks.
To the contrary, there is a mention, e.g., in (22) JP-A-3-259439 and (23) JP-A-4-253752 that existence of some amount of volatile components in a resin is effective for suppression of silver streaking on molding.
On the other hand, it has been proposed to add a stabilizer to a methacrylic resin to prevent pyrolysis, thermal discoloration or oxidation discoloration as disclosed in JP-A-59-15444, JP-A-4-216806, and JP-B-57-9392.
Nevertheless, none of the above-described techniques is to solve the discoloration problem observed in a light conducting plate made of a methacrylic resin by injection molding, i.e., to eliminate luminance unevenness or color unevenness in a side-lighted type surface lighting unit.
Characteristics generally required of materials for light conducting plates include high transmission, colorlessness, freedom from foreign matter, high heat resistance, and, in the case of injection molding, satisfactory moldability and satisfactory mold release properties. Specifically, the materials include such transparent materials as glass, methacrylic resins, styrene resins, and polycarbonate resins. Actually methacrylic resins which have excellent optical characteristics and are lighter than glass are used frequently.
In recent years inexpensive mass production of a light conducting plate by injection molding has prevailed as a reflection of the drastic increase of demand of laptop or notebook word processors or computers. As to the form of a light conducting plate, a thin wedge type is increasing for the purpose of minimizing power consumption.
While a methacrylic resin has excellent transparency and thereby exhibits satisfactory performance as light conducting plate, the requirement for further improvement in transparency has been getting more strict according as full color liquid crystal displays are spreading. Compared with conventional lighting units, a light conducting plate has a very long length through which light should pass so that only slight coloration of the material causes color unevenness.
Referring to FIG. 1, since light ray a, which is reflected near the light source, has passed through the material over a relatively short distance, it is insusceptible to the influence of the color of the material and is emitted toward a viewer side as having a color near to the light source's. On the other hand, light ray b, which is emitted from the position far from the light source, is susceptible to the influence of the color unevenness of the material and tends to assume a different color from that of the light source while it is passing through the material over a long distance. It follows that the color of scattered light of a surface lighting unit varies within the same light emitting surface depending on the distance from the light source, which leads to color unevenness.
Taking, for instance, a 3 mm thick plate produced by general injection molding of a methacrylic resin having a sufficiently increased purity, the yellowness index (YI) in the transmission direction is about 0.7 to 1, which means satisfactory transparency giving rise to no problem in usual use. However, if a similarly injection molded article has a light pass length exceeding 220 mm, a delicate color of the material itself would be accumulated, sometimes reaching the yellowness index of about 5. If a material having such a yellowness index is used as a light conducting plate of a side-lighted type surface lighting unit, the yellowness becomes stronger with the distance from the light source, resulting in color unevenness as a whole. In particular, this would be a cause of reduction in color reproducibility in a full color liquid crystal display. It is desirable for a full color liquid crystal display to have a backlight whose color unevenness is within a range of from 0.001 to 0.005 as expressed in terms of a difference between the maximum and the minimum on both x and y chromaticity coordinates according to a CIE calorimetric system. It is more desirable, as a matter of course, that no change occurs on both x and y coordinates.
In order to eliminate such yellowing, improvement in tone by color toning has been attempted. For example, a method called bluing consisting in addition of a blue pigment has been widely employed to correct the above-described delicate yellowness. Although this method produces an effect of reducing color unevenness due to yellowing, it involves reduction in transmission. Therefore, application of the bluing technique to a light conducting plate results in reduction of light utilization (i.e., reduction of luminance). There is a method for tone improvement which comprises adding a milky white pigment, such as titanium oxide, and a fluorescent brightening agent to a methacrylic resin to be used as a cover of a fluorescent tube. However, a light conducting plate made of such a milky white material not only causes excessive light scattering in the vicinities of the light source, leading to local excessive brightness, but also fails to transmit light to the part far from the light source. As a result, light emitted from the light conducting plate becomes non-uniform. Thus, materials for light conducting plates must be transparent as stated above.
As another approach to an improved luminance, a method of incorporating a fluorescent brightening agent alone into a methacrylic resin has been suggested, as disclosed in (24) JP-A-62-32488 and (25) JP-A-64-42686, in which fluorescence excited by ultraviolet rays emitted from a light source is utilized for improvement of luminance. This method is deemed to be unsuitable for improving luminance unevenness and color tone because a blue tint becomes strong, depending on the intensity of the ultraviolet light of the fluorescent tube. Addition of a fluorescent brightening agent to a transparent resin for a fluorescent tube cover is disclosed in (26) JP-B-6-3682. This method aims at cutting the ultraviolet light of a fluorescent tube and produces no effect on the subject matter of the present invention, improvement of tone or light utilization. In other words, the method has no effect of converting incident light in the ultraviolet region to light in the visible region.
Accordingly, an object of the present invention is to eliminate discoloration of an injection molded methacrylic resin light conducting plate, that is, to increase the luminance, to eliminate luminance unevenness and to improve color tone of a side-lighted type surface lighting unit.
Methacrylic resins are used as material for light conducting plates for their superiority in transparency to other molding materials. With reference to resins for optical disks, (20) JP-B-7-132598 states that a colorless and transparent methacrylic resin having a high optical purity can be obtained by specifying the molecular weight, pyrolysis index and yellowness index of a methacrylic resin and the volatile content and fine foreign matter particle content in the molding material. However, where a light conducting plate is produced by injection molding, the methacrylic resin disclosed is still insufficient. That is, the resin tends to undergo pyrolysis, thermal discoloration or oxidation discoloration during molding by the influences of the temperature distribution in an injection cylinder, the shape of the cylinder, and entrained air oxygen from the hopper, etc., only to provide colored light conducting plates. From this standpoint, the aforesaid method concerning a molding resin for optical disks which comprises actively reducing the monomer content in a molding resin, as suggested in (21) JP-A-58-154751, is not favorable because of involvement of the influence of oxygen due to entrainment of air on feeding from a hopper, etc. The effect of the aforesaid technique of (22) JP-A-3-259439 and (24) JP-A-4-253752, which comprises incorporating volatile components into a resin to some extent, resides in inhibition of silver streaks during molding and differs from the inhibitory effect on discoloration as aimed in the present invention.
Variations of the size or kind of a molding machine, the screw design, and other molding conditions make it technologically difficult to completely control heat generation, air entrainment, etc. of a resin through some mechanical means.
The above-described addition of a stabilizer to a methacrylic resin to prevent thermal discoloration or oxidation discoloration thereby to improve the tone of a light conducting plate, as proposed in JP-A-59-15444, JP-A-4-216806, and JP-B-57-9392, is insufficient in effect and rather brings about coloration.
As has been reviewed, the conventional techniques fail to provide a colorless transparent light-conducting plate by injection molding, i.e., a surface scattering lighting unit which satisfies the requirements demanded for use as a backlight of color liquid crystal displays, namely, high luminance, freedom from color unevenness, and uniform light transmission and scattering.
In order to obtain a high luminance and colorless light conducting plate it is necessary to specify the molecular weight of a methacrylic resin to be used and to inhibit coloration due to thermal oxidation on injection molding.
In order to obtain a light conducting plate having a small thickness and a precise uneven pattern by injection molding it is necessary to improve fluidity of an acrylic resin. To this effect, it is a generally followed technique to reduce the molecular weight of a resin or to copolymerize a comonomer having an internal plasticizing action. A resin having a merely increased comonomer content, while exhibiting improved fluidity, has reduced heat resistance and is unsuitable. A resin having a merely decreased molecular weight, while exhibiting improved fluidity, has poor mechanical strength.
An acrylic resin exhibits relatively strong adhesion to metal. If the adhesion to an injection mold is too strong, the molded article suffers from defects such as cracks, breaks, or surface roughness, upon release from the mold. Injection molding into a light conducting plate is desirably carried out at as low a molding temperature as possible in order to prevent deterioration of tone due to the foreign matter or to prevent thermal discoloration. However, fluidity of a molding resin in a mold would be reduced at low molding temperatures, which necessitates increase of the injection pressure to avoid short molding. As a result, the adhesion between a molded article and a mold becomes stronger, giving rise to the aforesaid problem of cracking. Accordingly, in order to achieve low-temperature molding, improvement of fluidity of the resin is required. However, excessive molecular weight reduction for improving the fluidity leads to reduction in mechanical strength, which causes cracking on release from a mold. In other words, in order to obtain a thin light conducting plate by injection molding at a possible lowest temperature, it is preferable to specify the molecular weight of a methacrylic resin to be used and to improve release properties of the resin.
Known techniques for improving the release properties of acrylic resins include addition of at least one of a polyhydric alcohol fatty acid ester, a monohydric alkyl alcohol, a fatty acid, a fatty acid amide, and a fatty acid metal salt (JP-A-61-73754), addition of a glycerol higher fatty acid ester and a saturated aliphatic alcohol (JP-A-1-294763), addition of at least one of glycerol stearate, glycerol behenate, and a fatty acid alkyl ester (JP-A-2-115255), and addition of a glycerol higher fatty acid ester whose alkyl moiety has a specific carbon atom number distribution (JP-A-4-53860 and JP-A-4-253752). The resin compositions disclosed still have insufficient fluidity or release properties or exhibit low heat resistance for use as resins for light conducting plates.
Accordingly, another object of the present invention is to solve such problems as poor mechanical strength (warpage or cracks on release) and poor appearance (e.g., silver streaks) as well as unevenness in color and luminance due to discoloration.